Method of and means for operating geothermal wells

ABSTRACT

A geothermal well is operated by continuously pumping spent geothermal fluid into the well and continuously adding concentrated acid to the spent geothermal fluid, such that the concentration of acid in the fluid is in the range of 0.5% to 5%, and occasionally to 10%. Over a period of time, the spend geothermal fluid flowing through the geological formations of the geothermal field carries the dilute acid into the formations for dissolving rock and increasing the permeability thereof. If it is desired to decrease the permeability of the formations, components of drilling mud or other suitable material, are continuously added to the spent geothermal fluid.

DESCRIPTION

1. Technical Field

This invention relates to a method of and for operating a geothermalwell, and more particularly, for increasing its flow rate.

2. Background Art

As a geothermal field ages by reason of long term extraction of hotgeothermal fluid from geological formations in the field, and long termreinjection of spent heat depleted geothermal fluid into the field, tworesults often occur: 1) a reduction in the mass flow rate andtemperature of hot geothermal fluid from production wells in the field;and 2) an increase in the pressure required to reinject spent geothermalfluid into injection wells in the field. Both problems apparently resultfrom reductions in the permeability of the geological formations in thefield.

Another common problem that often occurs in an operating geothermalfield is coupling between an injection well and a production wellwhereby the heat depleted geothermal fluid pumped into the injectionwell begins to adversely affect the temperature of the hot geothermalfluid extracted from a nearby production well. The exact mechanism thatcontrols these results is not entirely understood at this time, but itis presently believed that the problems are related to the permeability,or changes in the permeability, of the geological formations in whichthe wells are located. Such changes apparently are caused by long-termextraction of hot geothermal fluid from, and long-term injection ofspent geothermal fluid into, the geological formations of a field.

In order to increase the rate at which spent geothermal fluid can bedisposed of in an injection well, it is conventional to carry out whatis termed "acidizing" the well. Such approach is also used forincreasing the flow rate of geothermal fluid from a production well.This is a procedure by which concentrated acid is applied to a stratumwhose location and nature is determined from geological studies for thepurpose of increasing the permeability of the stratum by the dissolvingaction of the acid on the rock in the stratum. The procedure, which istime consuming, costly, and dangerous, requires the injection well to betaken out of operation.

The first step after operation is terminated is to place rubber seals inthe well at the upper and lower limits of the stratum to be treated.These placements require the use of a rig having drilling pipes forinserting and withdrawing the rubber seals; and usually, a considerableinvestment in time and labor is involved. Next, a pipe is run down thewell from the surface to the upper seal for gaining access to the regionof the well between the rubber seals. An acid solution of 10-20%concentration is then pumped through the pipe into the volume betweenthe seals for a period of from 1 to a few hours. Both water and acidmust be trucked to the well-head for this purpose; and a considerablevolume of liquid is required for this operation. The acid solutionpercolates through the stratum defined by the seals and dissolves someof the rock therein increasing the permeability of the stratum. Finally,the rubber seals are removed by the rig, and operation can be resumed.

This is a batch approach to increasing the permeability of a stratum. Itis expensive because the well being treated must be taken out ofoperation, and testing of the efficacy of the treatment can be done onlyby closing the well and resuming operations. This results in a tendencyto over-treat a well which further increases the cost in money and time.

Concerning the problem of leakage between an injection well and aproduction well, no practical solution is presently available. All ofthese problems are beginning to seriously affect existing geothermalfields which still contain a significant amount of energy extraction ishighly desirable.

It is therefore an object of the present invention to provide a new andimproved method of and means for operating geothermal wells toselectively modify the permeability of geological formations in whichboth injection and production wells are located.

BRIEF DESCRIPTION OF THE INVENTION

According to the present invention, spent geothermal fluid iscontinuously applied to a geothermal well, using a pump, for example,and sufficient concentrated acid is continuously added to the spentgeothermal fluid such that the concentration of acid in the fluid is inthe range 0.5% to 10%, but preferably, that concentration is not greaterthan 5%. Normally, this operation will occur over a period of timemeasured in days or weeks, rather than hours as in the case of the priorart, and is thus a continuous rather than a batch process. Thegeothermal fluid referred to in this specification is mostly geothermalliquid. Even so, the term should be taken to include steam, wherein thesteam may be present with liquid, or steam alone. Preferably, somequantities of substances such as inhibitors suitable for reducingcorrosive effects of the acid solution on the well casing or liner maybe added to the spent geothermal fluid, particularly to control the pHof the dilute acid solution. Usually, the pH will be increased by theadditives.

In accordance with the invention, the natural flow of the geothermalfluid through the various strata of the geological formation in whichthe well is located, will carry the dilute acid to those portions of theformation susceptible to being dissolved. The integrated effect of theexposure of the rock in the formation to dilute acid over a relativelylong period of time is to slowly increase the permeability of thevarious strata throughout the entire field in which the spent geothermalfluid flows. This continuous process, which is from the batch process ofthe prior art, does not require suspension of operation of an injectionwell, and ensures the application of an acid solution to whatever stratathe flow of spent fluid passes, thereby applying the dilute acid to thevarious strata. The long term effect of this approach is to increase thepermeability of various strata thereby decreasing the resistance of theinjection well to the flow of spent geothermal fluid.

The acid solution may be added to the spent geothermal fluid before thelatter enters the well. Preferably, however, the acid solution is addedto the fluid at a predetermined depth in the well consistent with thelocation of the slotted liner in the well through which the geothermalfluid flows into the surrounding rock formations. This latter operationis carried out by inserting a conduit into the injection well such thatthe lower end of the conduit is located below the upper unslottedmetallic casing of the well. This has the advantage of protecting theupper casing from being corroded by the dilute acid. A solution ofconcentrated acid is then pumped into the upper end of the conduit sothat the solution exits the lower end below the casing and is carried bythe spent geothermal fluid into the geothermal formations in which thewell is located.

The present invention also involves a method for increasing flow from aproduction well. In such case, the flow of hot geothermal fluid from theproduction well is terminated, and spent geothermal fluid is divertedfrom a nearby injection well and forced into the production well. Acidsolution is added to the spent geothermal fluid in the manner describedabove, preferably by piping the dilute acid to a level below theunperforated casing. Over a period of several days or weeks, the diluteacid will increase the permeability of the rock formations penetrated bythe spent geothermal fluid carrying the dilute acid. At the end of thetreatment period, production of the well may be resumed with anincreased rate of flow.

The present invention also involves a method for reducing leakagebetween an injection well that receives spent geothermal fluid, and aproduction well that produces high temperature geothermal fluid.According to the present invention, a conduit is inserted into theinjection well to a depth consistent with the stratum in which leakageis believed to occur between the injection well and the production well.Components of drilling mud, or other material suitable for reducing thepermeability of the stratum can be pumped into the conduit for effectinga flow of the mud or other material into the stratum: reducing itspermeability and suppressing further leakage. Alternatively, drillingmud components, or other suitable material, can be added to the spentgeothermal fluid before it enters the well. In such case, the mud orother material flows with the spent geothermal fluid as the latter flowsalong its usual paths into the stratum surrounding the well locallyreducing its permeability, thus reducing the underground fluid flowvelocity, and suppressing further leakage. Furthermore, such methods canbe used to merely reduce permeability of certain formation stratasurrounding the well.

In addition, the present invention also involves a method for developinga new geothermal injection or production well once drilling iscompleted. According to the present invention, preferably spentgeothermal fluid form a nearby injection well is diverted to the drilledwell and forced into the well. Acid solution is added, in the mannerdescribed above, to the geothermal fluid for the purpose of increasingthe permeability of rock formations surrounding the well. At the end ofthe treatment period, operation of the well as an injection orproduction well, as the case may be, can be commenced.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are shown by way of example in theaccompanying drawings wherein:

FIG. 1 is a schematic block diagram showing one aspect of the presentinvention wherein acid solution is continuously added to the spentgeothermal fluid being injected into a well in a geothermal field;

FIG. 2 is a schematic block diagram showing another embodiment of thepresent invention for treating an injection well or a production wellwith dilute acid in a continuous process;

FIG. 3 is a further embodiment of the present invention;

FIG. 4 is a still further embodiment of the present invention forreducing leakage between an injection well and a production well; and

FIG. 4A is another embodiment of the present invention for reducingleakage between an injection well and a production well.

DETAILED DESCRIPTION

Referring now to FIG. 1 of the drawings, reference numeral 10 designatesapparatus according to the present invention for adding concentratedacid to spent geothermal fluid that is pumped into a rejection well.That is to say, source 12 represents the equipment that operates on hotgeothermal fluid, extracts heat therefrom, and produces spent, or heatdepleted geothermal fluid that must be disposed of. Pump 14 may beprovided for pressurizes the spent fluid before the latter is deliveredto well-head 16 via pipe 18. Such fluid is applied to injection well 20,and the pressure is sufficient to force the fluid down the well andthrough the openings in slotted liner 22 that constitutes the lowerportion of the well. The fluid that exits the well enters into and flowsthrough permeable rock in the geological formation in which the well islocated until the pressure is dissipated.

As the well ages, the permeability of the rock formations apparentlydecrease, perhaps due to the precipitation of the mineral rich fluidwhich is cooled by contact with the rock formations. Furthermore, itcould well be that the permeability of the rock formations surroundingthe well was initially insufficient. In any event, experience has shownthat injection pressure must be increased with time to maintain the samemass flow of fluid. The first approach to ameliorating this problem isshown in FIG. 1 wherein concentrated acid, preferably hydrochloric acid,in source 24 is metered at 26 into the flow of fluid in pipe 18 suchthat the concentration of acid in the fluid is in the range 0.5% to 10%.Preferably, the concentration does not exceed about 5%. Small quantitiesof additives, such as inhibitors present in container 25, are fed intothe spent geothermal fluid to reduce corrosive action of the acidsolution on the well casing or lining by controlling the pH of thediluted acid.

As a consequence, the spent geothermal fluid carrying the dilute acidflows through slotted liner 22 along its usual paths into the rockformation surrounding the well. Wherever dissolvable rock is encounteredin these paths, the dilute acid reacts with the rock increasing itspermeability.

The treatment continues over a period of time measured in days and weeksas the dilute acid slowly but effectively continues to dissolve rock inthe flow path of the fluid. The effect on the surrounding rockformations is the integration over a long period of time of dilute acidacting on the rock; and eventually, the permeability of the rock will besufficiently increased to result in a reduction in the pressure neededin order to dispose of the spent geothermal fluid.

The actual amount of time will, of course, depend on the nature of thegeological formation in which the well is located; but it is presentlycontemplated that a time period in excess of 24 hours will be required,and probably of the order of magnitude of a week. In this sense,therefore, the present invention involves a continuous treatment of theentire well without interrupting its operation, as distinguished fromthe conventional treatment which is a batch process that involvesshutting down the well and acidizing only a preselected stratum for onlya few hours.

Because the dilute acid in the spent geothermal fluid may have acorrosive effect on the well head equipment and the upper casing of thewell above the lower slotted liner 22A, the arrangement shown in FIG. 2is preferred when the concentration of acid in the fluid exceeds about1%. As shown in FIG. 2, well 20A is treated, according to the presentinvention, by metering concentrated acid into the spent geothermal fluidat a location below casing structure 28 of the well. To effect thisoperation, conduit 29 is inserted into the well to a depth consistentwith the location at which acid treatment is to begin. This depth isdetermined by an analysis of geological information associated with thewell which is beyond the scope of this specification, suffice it to saythat conduit 29 is inserted into the well, preferably such that itslower end 30 is located at the desired depth, and its upper end 31terminates in a connection to source 24A.

Metering pump 32 pumps concentrated acid from source 24A to lower end 30of the conduit into the well at a rate such that the concentration ofacid in the spent geothermal fluid at the outlet of the conduit nogreater than about 10%, but preferably no greater than about 5%. Thetreatment region will be that portion of the geological field affectedby the flow of fluid from the well below the end 30 of the conduit.

A further embodiment of the invention is shown in FIG. 3 wherein tworeactants are injected into the geothermal fluid, the reactantsproducing a reaction product when they mix. In a manner analogous to theoperation of the embodiments described above, the flow of geothermalfluid into the geological formation surrounding the well serves to carrythe product into the rock formations for changing the permeability ofthereof. Depending upon the nature of the product, the permeability maybe increased or decreased.

Specifically, geothermal fluid is pumped down well 20B in the usualmanner; but, as shown in FIG. 3, conduit 31B extends from the surface ofthe well to a level below casing 28B in the vicinity of the upperportion of slotted liner 22B. Conduit 31B comprises a pair of pipes 32,33 whose top ends 34, 35 are connected respectively to differentreactant sources 36, 37. The reactants in the sources are of the typethat react to form a reaction product when mixed.

In operation, conduit 31B is inserted into the well, and metering pumps38 inject the two reactants into the well at the level determined by thelower end of the conduit. The two reactants mix together when they exitfrom pipes 32, 33 producing a product that is carried by the flowinggeothermal fluid into the surrounding rock formation. As before, thetime period over which the product is produced is measured in days andweeks, depending on the nature of the product.

Furthermore, the present invention also contemplates injectingcomponents of drilling mud or other suitable materials into an operatinginjection well for the purpose of decoupling the well from a productionwell. This approach is illustrated in FIG. 4 where conduit 31C islocated in injection well 20C which is in the vicinity of productionwell 40 from which hot geothermal fluid is withdrawn. The lower end ofthe conduit is located at a level consistent with a leakage path 41 thatconnects well 20C to well 40. Components of drilling mud, produced atthe surface of the injection well, are pumped into the injection wellthrough conduit 31C and flow though the slotted liner into the rockformation following the path of least resistance which is the leakagecoupling to the production well. The result is that the drilling mudcomponents, or other material decrease the local permeability of therock formations through which they percolate, reducing underground fluidflow velocity and achieving decoupling the injection well from theproduction well. If preferred, decoupling of an injection well from aproduction well also can be achieved using an arrangement similar tothat illustrated in FIG. 1. As shown in FIG. 4A, components of drillingmud, or other suitable materials, present in container 24D can be addedto spent geothermal fluid flowing in pipe 18D prior to the entry of thefluid into the injection well. Furthermore, such method can be usedmerely to reduce permeability of certain formation strata surroundingthe well.

Additionally, the present invention also can be used for developing newgeothermal injection or production wells once drilling is completed.Methods shown in FIGS. 1-3 can be used for adding acid solutions, orother suitable chemicals, to spent geothermal fluid diverted from aninjection well and forced into the newly drilled well. The fluidcarrying the dilute acid will flow into rock formations surrounding thewell, and the dilute acid will react with the formations increasingtheir permeability. Subsequently, at the end of the treatment period,operation of the well as an injection or production well, as the casemay be, can be commenced.

At present, it is envisaged that the present invention will be carriedout to change the permeability of rock formations at depths ranging from100-2000 m. However, in actual fact, the invention can be used forchanging the permeability of rock formations at any depth.

The advantages and improved results furnished by the method andapparatus of the present invention are apparent from the foregoingdescription of the preferred embodiment of the invention. Variouschanges and modifications may be made without departing from the spiritand scope of the invention as described in the appended claims.

We claim:
 1. A method for operating a geothermal well located in ageological formation, said method comprising the steps of:a)continuously supplying spent geothermal fluid to the well for effectingdissipation of the spent fluid in the geological formation; and b)continuously adding concentrated acid to the spent geothermal fluid inthe well such that the concentration of acid in the fluid is sufficientto increase the permeability of said geological formation.
 2. A methodaccording to claim 1, wherein the acid solution is added to the fluidbefore the latter enters the well.
 3. A method according to claim 1,wherein the well is an injection well.
 4. A method according to claim 1,wherein the well is a production well.
 5. A method according to claim 1wherein the concentration of acid in the fluid is not greater than about5%.
 6. A method according to claim 1 wherein the spent geothermal fluidis applied to the well using a pump.
 7. A method according to claim 1wherein said acid is hydrochloric acid.
 8. A method according to claim 1wherein the concentration of acid in the fluid is in the range of 0.5%to 10%.
 9. A method for operating a geothermal well comprising the stepsof:a) continuously applying spent geothermal fluid into the well; and b)continuously adding sufficient concentrated acid to the spent geothermalfluid in the well such that the concentration of acid in the fluid is inthe range of 0.5% to 10%; and c) wherein the acid solution is added tothe fluid at a predetermined depth in the well.
 10. A method forreducing leakage between an injection well that receives spentgeothermal fluid and a production well that produces hot geothermalfluid, said method comprising the steps of:a) inserting a conduit intothe injection well to a depth consistent with the stratum at whichleakage occurs between the injection well and the production well; andb) pumping components of drilling mud into the conduit for effecting aflow of mud into said stratum for suppressing further leakage betweenthe injection well and the production well.
 11. A method for operating ageothermal well having an upper casing and a lower slotted liner andlocated in a geothermal field having various geological formations, saidmethod comprising the steps of:a) applying spent geothermal fluid tosaid well; b) inserting a conduit into the well such that the lower endof the conduit is located below said upper casing; and c) pumping anacid solution into the top of the conduit so that the solution exits thelower end of the conduit below said casing, and is carried by the spentgeothermal fluid into said geological formations, the acid solutionhaving a concentration sufficient to increase the permeability of thegeological formations into which the acid solution is carrier.
 12. Amethod according to claim 11 wherein the concentration of acid in thefluid is in the range 0.5% to 10%.
 13. A method according to claim 11,wherein said well is an injection well and the pumping of the spentgeothermal fluid and the solution takes place over a period of timegreater than 24 hours.
 14. A method according to claim 11, wherein saidwell is a production well and the pumping of the spent geothermal fluidand the solution takes place over a period of time greater than 24hours.
 15. A method according to claim 7 wherein the spent geothermalfluid is applied to the well using a pump.
 16. A method for operating ageothermal well having an upper casing and a lower slotted liner andlocated in a geothermal field having various geological formations, saidmethod comprising the steps of:a) applying spent geothermal fluid tosaid well; b) inserting a conduit into the well such that the lower endof the conduit is located below said upper casing; c) pumping a solutioninto the top of the conduit so that the solution exits the lower end ofthe conduit below said casing, and is carried by the spent geothermalfluid into said geological formations; and d) wherein said solution isan acid.
 17. A method according to claim 16 wherein the concentration ofacid in the fluid is not greater than about 5%.
 18. A method foroperating a geothermal well having an upper casing and a lower slottedliner and located in a geothermal field having various geologicalformations, said method comprising the steps of:a) applying spentgeothermal fluid to said well; b) inserting a conduit into the well suchthat the lower end of the conduit is located below said upper casing; c)pumping a solution into the top of the conduit so that the solutionexits the lower end of the conduit below said casing, and is carried bythe spent geothermal fluid into said geological formations; and d)wherein the solution includes components of drilling mud.
 19. Apparatusfor operating a geothermal well comprising:a) source of spent geothermalfluid; b) means for applying said fluid into said well; and c) means foradding acid to said fluid such that the concentration of acid in thefluid is in the range 0.5% to 10%.
 20. Apparatus according to claim 19wherein said means for adding is constructed and arranged so that thesolution is added to the fluid only at a predetermined depth in thewell.
 21. Apparatus according to claim 20 wherein said means for addingincludes a conduit in said well extending from the surface to saidpredetermined depth, said conduit having a top end at the surface of thewell into which said acid is added, and a bottom open end at which saidpredetermined depth for effecting entry of said solution into saidfluid.
 22. Apparatus according to claim 19 wherein said means forapplying said fluid to the well is a pump.
 23. Apparatus according toclaim 19 wherein said acid is hydrochloric acid.
 24. A method fordeveloping a new geothermal well which has been drilled, said methodcomprising the steps of:a) continuously applying spent geothermal fluiddiverted from an injection well to the new well; and b) continuouslyadding concentrated acid to the spent geothermal fluid in an amountsufficient to achieve an acid concentration in the range 0.5% to 10%.25. A method according to claim 24 wherein the geothermal well is aninjection well.
 26. A method according to claim 24 wherein thegeothermal well is a production well.
 27. A method according to claim 24wherein the concentration of acid in the fluid in not greater than about5%.
 28. A method according to claim 24 wherein the spent geothermalfluid is applied to the well using a pump.
 29. A method for operating ageothermal well comprising the steps of:a) continuously applying spent,geothermal fluid to the well; and b) continuously adding components ofdrilling mud to the spent geothermal fluid for decreasing thepermeability of geological formations surrounding the well.